Such calenders for the production of fiber reinforced rubber packing sheets have been built for many years. In such calenders, the heated cylinder, on account of its large diameter of at least 1,000 mm. or more, is made hollow and serves simultaneously as the steam drum for heating of the outer surface of the cylinder. The steam is admitted through the central bore of one of the stub shafts and the condensate is again discharged through a tube extending centrally in the stub shaft.
The heating with steam has serious disadvantages. For one thing these cylinders must meet special structural requirements in order to comply with testing regulations. On the other hand, the surface temperature of such steam drums cannot be regulated fast enough or precise enough for present day technological processes.
Hence it was proposed about fifteen years ago in DE-AS 23 21 367 to heat the hot cylinder of a packing sheet calender with hot water which is led through bores in the casing of the hot cylinder parallel to its axis. For this purpose, the hot cylinder has a hollow through shaft from which radial channels in the ends of the cylinder extend to the axis-parallel bores in the casing. However, this proposal has never been realized because its construction encountered insuperable difficulties. These difficulties arose on the one hand in the creation of a mechanically tight connection between the shaft and the cylinder end and on the other hand in sealing difficulties. The very massively constructed cylinder casing, on the ground of stability, has a considerable weight, which on rapid braking of the shaft leads to a considerable torsional moment where the end of the cylinder is connected with the shaft. As in these same places radial channels extend out from the hollow shaft in the cylinder ends, there are insurmountable construction difficulties in these cases in obtaining a sufficiently stable connection of the cylinder ends on the hollow shaft. Moreover, at these same places there are also sealing difficulties because a spatially very close arrangement of the radial channels is required, of which the one supplies water to the axis-parallel channels of the casing and the other discharges the water. Thus on each end supply channels and also discharge channels are arranged, if the outer surface of the cylinder is to be uniformly heated, for example through a heating channel arrangement as shown in DE-PS 23 15 669.
The hot cylinders of packing sheet calenders cannot be made of solid material, because their mass would be so great that they could not be braked fast enough in case of emergency or also for an economical production operation, the high braking forces and the weight of the cylinder rendering the device technically uninteresting. Hence a radial channel arrangement according to DE-PS 23 15 669 is no more possible than a cylinder construction according to U.S. Pat. No. 2,936,158, in which a massive cylinder with recesses in its outer surface is surrounded by a schrunk-on cylindrical casing. This construction form is also prohibited on account of the unequal heating of the casing surface because here in the region of one end of the cylinder the heating medium enters while at the other end the heating medium is exhausted.
On the other hand, the hot cylinder of a packing sheet calender is not so easily constructed as in DE-PS 838 856 for the drying cylinder of a paper machine because the pressing forces on the outer face of the cylinder in a packing sheet calender are far greater and the cylinder casing requires a high hardness and resistance to wear.